The present invention relates to a method for producing a chromium-iron-based thermal spraying material employed in corrosion-resistant members.
Spray coating is a technique widely utilized in industry for improving the quality of surfaces of substrates, and is employed for imparting corrosion resistance, wear resistance, and heat resistance to members of airplane engines, power-generating gas turbines, paper-making machines, iron or steel rolls, and other products. In spray coating, a thermal spraying material, which is generally in the form of powder, is instantaneously melted by use of a high-temperature heat source, and the thus-melted material is solidified and deposited onto the surface of an object, to thereby form a coating on the object. In accordance with the heat source employed for melting a thermal spraying material, spray coating is roughly classified into a combustion gas type in which a fuel such as propylene, acetylene, or kerosene is combusted in the presence of air or oxygen; and a plasma spraying type employing, for example, argon, hydrogen, helium, or nitrogen. These types are appropriately employed in accordance with the purpose of spray coating.
As compared with other coating methods, such as plating, spray coating is more advantageous in that (1) coating is formed at a high rate, (2) a substrate undergoes minimal thermal deformation, since a plating frame is not brought into direct contact with the substrate, and (3) the size of a substrate is not limited by the dimension of a plating bath, thereby facilitating coating of a substrate having a large area. Therefore, spray coating is used in practice for preventing corrosion or rusting of large-sized structures, such as steel towers, iron bridges, steel frames, ships, and containers employed in chemical industries.
Examples of thermal spraying materials which are used in practice include aluminum, zinc, zinc-aluminum alloys, chromium-iron-based alloys, nickel-chromium alloys, CoCrAlY alloys, and NiCrAlY alloys. Of these, chromium-iron-based alloys are employed for providing corrosion resistance. For example, when a chromium-iron-based alloy film is formed through spray coating on the inner wall of a sodium-sulfur battery positive electrode container, the battery container can be protected from a corrosive active substance.
Conventional chromium-iron-based alloy thermal spraying materials are generally produced by grinding a chromium-iron-based alloy ingot, and subjecting the thus-ground ingot to classification.
A grinding machine such as a vibration mill or an attritor is generally employed for production of chromium-iron-based alloy thermal spraying materials. A chromium-iron-based alloy thermal spraying material having a particle size of about some hundreds of xcexcm is easily produced through grinding. However, a chromium-iron-based alloy thermal spraying material having a particle size of 100 xcexcm or less is difficult to produce through grinding, due to toughness of chromium-iron alloy. Excessive grinding causes wear of the interior of a grinding machine, along with contamination of the resultant thermal spraying material with impurities. When a chromium-iron-based thermal spraying material having a particle size of 100 xcexcm or less is produced through a conventional grinding method, particles of the resultant thermal spraying material tend to have a flat shape. Therefore, such thermal spraying material is not suitable for spray coating, which requires fluidity of particles.
The present inventors have performed studies on grinding treatment of a chromium-iron-based alloy, and have found that when a chromium-iron alloy is subjected to heat treatment, the alloy becomes brittle, and that when the resultant alloy is subjected to grinding treatment, fine powder can be formed, and the thus-formed powder has a shape suitable for spray coating. The present invention has been accomplished on the basis of this finding. Accordingly, the present invention provides the following.
(1) A method for producing a chromium-iron-based thermal spraying material characterized by comprising subjecting a chromium-iron-based alloy to heat treatment, and subsequently subjecting the heat-treated alloy to grinding treatment.
(2) A method for producing a chromium-iron-based thermal spraying material according to (1), wherein the chromium-iron-based alloy comprises an alloy having a chromium content of 60 to 95 mass %.
(3) A method for producing a chromium-iron-based thermal spraying material according to (1) or (2), wherein the chromium-iron-based alloy is in the form of particles having a size of 5 xcexcm to 1 mm in diameter.
(4) A method for producing a chromium-iron-based thermal spraying material according to any one of (1) through (3), wherein the heat treatment of the chromium-iron-based alloy is carried out in an atmosphere of hydrogen gas or an inert gas.
(5) A method for producing a chromium-iron-based thermal spraying material according to (4), wherein the inert gas is argon.
(6) A method for producing a chromium-iron-based thermal spraying material according to any one of (1) through (5), wherein the heat treatment of the chromium-iron-based alloy is carried out at 500 to 1,300xc2x0 C.
(7) A method for producing a chromium-iron-based thermal spraying material according to any one of (1) through (6), wherein the grinding treatment is carried out by use of an impact mill.
(8) A method for producing a chromium-iron-based thermal spraying material according to (7), wherein the impact mill includes a liner section having bar-like, wear-resistant members which are arranged to form a grating.
(9) A method for producing a chromium-iron-based thermal spraying material according to (8), wherein the bar-like, wear-resistant members are formed from any one material selected from among zirconia, yttria-stabilized zirconia, calcia-stabilized zirconia, magnesia-stabilized zirconia, alumina, alumina-zirconia, silicon carbide, silicon nitride, and a tungsten carbide-cobalt alloy.
(10) A method for producing a chromium-iron-based thermal spraying material according to any one of (1) through (9), wherein a thermal spraying material produced through the method has a particle size of 5 to 100 xcexcm.
(11) A method for producing a chromium-iron-based thermal spraying material according to any one of (1) through (10), wherein a thermal spraying material produced through the method has an apparent density as specified by JIS (Z2504) of 2.8 to 3.5 g/cm3.
(12) A chromium-iron-based thermal spraying material having a chromium content of 60 to 95 mass %, a particle size of 5 to 100 xcexcm, and an apparent density as specified by JIS (Z2504) of 2.8 to 3.5 g/cm3.